bpo-39274: Ensure Fraction.__bool__() returns a bool (GH-18017)

bpo-39274: Ensure Fraction.__bool__() returns a bool (GH-18017)

Some numerator types used (specifically NumPy) decides to not
return a Python boolean for the "a != b" operation. Using the equivalent
call to bool() guarantees a bool return also for such types.
(cherry picked from commit 427c84f13f7719e6014a21bd1b81efdc02a046fb)

Co-authored-by: Sebastian Berg <sebastian@sipsolutions.net>